lease-npv-calculator

HMRC-compliant Net Present Value (NPV) engine for UK property lease duty calculations — SDLT (England), LBTT (Scotland), and LTT (Wales).

Installation

npm install lease-npv-calculator

Usage

Standard lease NPV

import { calculateLeaseNPV, HMRC_DISCOUNT_RATE } from 'lease-npv-calculator';

const result = calculateLeaseNPV({
  annualRent: 50000,
  rentYears: [20000, 30000, 40000, 50000, 50000], // years 1–5 (optional)
  leaseStartDate: new Date('2024-01-01'),
  leaseEndDate: new Date('2034-01-01'),
  effectiveDate: new Date('2024-01-01'),
  bands: [
    { min: 0,      max: 150000,   rate: 0 },
    { min: 150000, max: Infinity, rate: 1 },
  ],
  proratePartialYear: true, // true for SDLT; false for LBTT/LTT
});

console.log(result.npv);       // NPV in £
console.log(result.totalTax);  // Tax due in £

With overlap relief (lease renewal)

import { calculateLeaseNPVWithOverlapRelief } from 'lease-npv-calculator';

const result = calculateLeaseNPVWithOverlapRelief({
  annualRent: 60000,
  leaseStartDate: new Date('2024-06-01'),
  leaseEndDate: new Date('2034-06-01'),
  effectiveDate: new Date('2024-06-01'),
  oldLeaseGrantDate: new Date('2014-06-01'),
  oldLeaseTerm: 11,
  oldLeaseRent: 50000,
  bands: [
    { min: 0,      max: 150000,   rate: 0 },
    { min: 150000, max: Infinity, rate: 1 },
  ],
});

console.log(result.npv);
console.log(result.totalTax);
console.log(result.overlapRelief); // overlap relief metadata

API

calculateLeaseNPV(input)

| Parameter | Type | Description | |---|---|---| | annualRent | number | Annual rent (£). Used for years 6+ and as fallback for years 1–5. | | rentYears | number[] | Optional rents for years 1–5 (index 0 = year 1). | | leaseStartDate | Date \| string | Lease commencement date. | | leaseEndDate | Date \| string | Lease expiry date. | | effectiveDate | Date \| string | Effective date of the transaction. | | bands | NpvTaxBand[] | Graduated tax bands. | | surchargeRate | number | Optional surcharge percentage (e.g. 2 for non-UK resident). | | discountRate | number | Defaults to 0.035 (HMRC mandated 3.5%). | | proratePartialYear | boolean | Prorate the final partial year. Default true. |

calculateLeaseNPVWithOverlapRelief(input)

Extends calculateLeaseNPV input with:

| Parameter | Type | Description | |---|---|---| | oldLeaseGrantDate | Date \| string | Grant date of the previous lease. | | oldLeaseTerm | number | Term of the previous lease in years. | | oldLeaseRent | number | Annual rent under the previous lease (£). |

Constants

import { HMRC_DISCOUNT_RATE } from 'lease-npv-calculator'; // 0.035

License

MIT